The reliability of toxic gas detectors is of great importance in many applications, especially when these instruments are used for ensuring the safety of personnel. Reliability is typically obtained by periodic checking of the instrument response to a test gas, however calibration test gases are typically supplied in large, bulky and expensive gas cylinders.
Potentially hazardous atmospheres are found in many locations, due to the presence of toxic gases, combustible gas mixtures or the excess or deficiency of oxygen concentration. Many types of gas detection instruments have been developed to provide a warning that the atmosphere contains potentially hazardous components, or to initiate remedial action. Examples of these gas detection instruments include the detection of combustible gases in coal mines, hydrogen sulfide in oil fields and water treatment plants, carbon monoxide in places ranging from steel mills to bedrooms, and oxygen in confined spaces, such as sewers. Within each gas detection instrument there are one or more gas sensors, whose function is to provide an electrical signal, which varies in response to the gas concentration.
Photoacoustic sensors may be used to detect sample gases based on the tendency of molecules of sample gases, when exposed to certain frequencies of radiant energy, to absorb the energy and reach higher levels of molecular vibration and rotation thereby to reach a higher temperature and pressure. When the radiant energy is amplitude modulated, the resulting fluctuations in energy available for absorption produce corresponding temperature and pressure fluctuations. A sensitive detector can be used to generate an electrical output representing the pressure fluctuations of the sample gases, which can be analyzed to evaluate properties or attributes of the sample gases.
Many existing photoacoustic sensors utilize commercial MEMs microphones to sense pressure waves on a flexible diaphragm by using capacitive pick off techniques to measure capacitance. Most MEMs microphones typically require the diaphragm to be at least 1.5 mm×1.5 mm×1 mm in size in order to attain a measurable capacitance.
In addition, most MEMs microphones usually require an additional area in order to accommodate an internal amplifier. The amount of additional area that is required to accommodate the internal amplifier typically depends on the complexity of the internal amplifier.
The voltage signals levels that are normally output from a MEMs microphone typically need to be enhanced in order to reach a sufficiently high level (i.e., millivolts) above the voltage signals levels that are associated with ambient noise.
Therefore, a need exists for a photoacoustic sensor that includes a relatively smaller microphone which is able to output voltage signals levels that are above the voltage signal levels that are associated with ambient noise. Making the microphone smaller would thereby permit the overall size and complexity of the photoacoustic sensor to be reduced.